As Warlokk says, you can't use Massive in Reason. You'll need a vst or rewire host to run both at the same time. Ableton, Cubase, FL Studio, Pro Tools, Logic (if you have a Mac)... I'm sure there is plenty more.
Massive is a killer softsynth, no doubt. Subtractor and Thor as mentioned above are then stand out synths from Reason but then I haven't really checked out version 5 yet.